Dissecting Dinosaur Pellets? Taphonomy of Modern Raptor and Carnivore Gastric Pellets Guides the Identification of Fossil Feeding Traces at Egg Mountain, a Dinosaur Nesting Site
FREIMUTH, William J.; Montana State University, Bozeman, MT. VARRICCHIO, David J.; Montana State University, Bozeman, MT.
Modern birds of prey (raptors), crocodilians, monitor lizards, snakes, and mammals are all terrestrial organisms that routinely regurgitate gastric pellets comprised of indigestible remains of prey items (typically bone and teeth). Despite the ubiquity of pellets in extant organisms, the fossil record of gastric pellets is sparse, particularly in Mesozoic terrestrial deposits. Those pellets that have been identified in the fossil record conform to the taphonomic characteristics of modern raptor pellets.
We describe two novel instances of potential gastric pellets and/or prey processing locales from Egg Mountain, a dinosaur nesting locality of the Upper Cretaceous Two Medicine Formation of Montana. The first amalgamation consists of a minimum of three individual metatherians (Alphadon) based on three pairs of disarticulated maxillae. In total, 70% of identifiable elements are cranial. A second amalgamation comprises a minimum of eight Alphadon and one lizard skull. Of the represented elements, 85% are cranial, including 29 dentary fragments (minimum of 13 dentaries), 12 maxillary fragments (minimum 11 maxillae), two braincases, and nine long bone shafts. Element breakage is sharp and terminated by matrix, indicative of pre-fossilization modification. The skewed distribution of cranial and tooth-bearing elements along with fragmentary long bone shafts resembles the distribution observed in modern raptor and carnivore pellets and contrasts element representation typical of hydraulic sorting. The size and composition of the amalgamations and their co-occurrence with two theropod egg types (Troodon and Continuoolithus) and shed teeth suggests producers were small theropods (e.g., Troodon or dromaeosaurids), though other potential producers (e.g., the large varanoid lizard, Palaeosaniwa) cannot be confidently excluded. Regardless, these feeding traces provide evidence for predation on marsupials and ecological activity at a dinosaur nesting site.